Current demands for high density and performance associated with ultra large scale integration in semiconductor devices require submicron features, increased transistor and circuit speeds, and improved reliability. Such demands require formation of device features with high precision and uniformity, which in turn necessitates careful process monitoring.
Known inspection techniques typically use imaging the articles with a large magnification on to a charge-coupled device (CCD) camera. The imaging technique requires the article to be illuminated. The brightness of the illuminating source is a key factor in the ability to speed the inspection by reducing the integration time of camera. As the patterns on wafers get smaller, it becomes necessary to use shorter wavelengths in order to be able to detect the patterns. This is due to the fact that the physical resolution limit depends linearly on the illumination wavelength and due to interference effects which require that the inspection be done at a wavelength similar to the one used in the lithographic process.
As the wavelengths get smaller, conventional incoherent light sources like filament lamps or gas discharge lamps do not have sufficient brightness, and the light sources of choice become short wavelength lasers. The coherence of the laser, together with the roughness and aberrations of the surfaces as well as the patterned article along the light path, creates an artifact known as speckle, which is a noisy pattern over the image of the article.
Speckle causes problems in detection of the surfaces of articles being inspected and causes false alarms because of the non uniformity of the light pattern hitting the detector. Detection accuracy is degraded. Also, images taken of inspected articles are degraded. The problem is an acute one in this type of article inspection, because the power provided by coherent light is essential, among other reasons, as a result of losses stemming from the detection process.
U.S. Pat. Nos. 7,463,352, 7,133,548, 6,924,891, 6,798,505, 6,587,194, 6,556,294, 6,466,315, 6,429,931, 6,369,888 and 6,268,093. all being incorporated herein by reference. provide some state Of the art speckle reduction methods and systems.
U.S. Pat. No. 7,449,673 discloses a system and a method for speckle reduction. It discloses, for example. a diffuser based solution. Diffusers are known to introduce their own speckle and complicate the speckle reduction solution.
When operating in the Ultra Violet (UV) region, and especially the deep Ultra Violet (DUP) region, the cost of optical components increases and various prior art speckle reduction solutions are either too expensive or inefficient.
As can be appreciated from the foregoing discussion, there is a need in the art for a method and system fir reducing speckle when inspecting articles using pulsed laser pulses at low wavelengths, including the deep UV region.